Continuously viewable, DC field-balanced, reflective, ferroelectric liquid crystal image generator

ABSTRACT

A reflection mode, ferroelectric liquid crystal spatial light modulating system, includes a light reflecting type spatial light modulator. The spatial light modulator has a light reflecting surface cooperating with a layer of ferroelectric liquid crystal light modulating medium switchable between first and second states so as to act on light in different first and second ways, respectively. A switching arrangement switches the liquid crystal light modulating medium between the first and second states and an illumination arrangement produces a source of light. An optics arrangement is optically coupled the spatial light modulator and the illumination arrangement such that light is directed from the source of light into the spatial light modulator for reflection back out of the modulator and such that reflected light is directed from the spatial light modulator into a predetermined viewing area. The optics arrangement includes a passive quarter wave plate positioned in the optical path between the light source and the spatial light modulator and in the optical path between the spatial light modulator and the viewing area. A compensator cell is also positioned in the optical path between the light source and the spatial light modulator and in the optical path between the spatial light modulator and the viewing area. The compensator cell has a layer of ferroelectric liquid crystal light modulating medium switchable between a primary and a secondary state so as to act on light in different primary and secondary ways, respectively.

This is a Continuation application of copending prior application Ser.No. 09/025,160, filed on Feb. 18, 1998, designated the United States,the disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to image generating systemsincluding reflective type, ferroelectric liquid crystal (FLC) spatiallight modulator (STM). More specifically, lie invention relates to alloptics arrangement including an FLC compensator cell for allowing thesystem to generate a substantially continuously viewable image whileDC-balancing the FLC material of both the SLM and the compensator cell.

FLC materials may be used to provide a low voltage, low power reflectivespatial light modulator due to their switching stability and their highbirefringence. However, a problem with FLC materials, and nematic liquidcrystal materials, is that the liquid crystal material may degrade overtime if the material is subjected to an unbalanced DC electric field foran extended period of time. In order to prevent this degradation, liquidcrystal spatial light modulators (SLMs) must be DC field-balanced.

Nematic liquid crystal materials respond to positive or negativevoltages in a similar manner regardless of the sign of the voltage.Therefore, nematic liquid crystals are typically switched ON by applyingeither a positive or negative voltage through the liquid crystalmaterial. Nematic liquid crystal materials are typically switched OFF bynot applying any voltage through the material. Because nematic liquidcrystal materials respond to voltages of either sign in a similarmanner, DC balancing for nematic liquid crystal materials may beaccomplished by simply applying an AC signal to create the voltagethrough the material. The use of an AC signal automatically DC balancesthe electric field created through the liquid crystal material byregularly reversing the direction of the electric field created throughthe liquid crystal material at the frequency of the AC signal.

In the case of FLC materials, the materials arc switched to one state(i.e. ON) 1 applying a particular voltage through the material (i.e. +5VDC) and switched to the other state (i.e. OFF) by applying a differentvoltage through the material (i.e. -5 VDC). Because FLC materialsrespond differently to positive and negative voltages, they cannot beDC-balanced in situations where it is desired to vary the ratio of ONtime to OFF time arbitrarily. Therefore, DC field-balancing for FLC SLMsis most often accomplished by displaying a frame of image data for acertain period of time, and then displaying a frame of the inverse imagedata for an equal period of time in order to obtain an average DC fieldof zero for each pixel making up the SLMs.

In the case of an image generating system or display, the image producedby the SLM during the time in which the frame is inverted for purposesof DC field-balancing may not typically be viewed. If the system isviewed during the inverted time without correcting for the inversion ofthe image, the image would be distorted. In the case in which the imageis inverted at a frequency faster than the critical flicker rate of thehuman eye, the overall image could be completely washed out and all ofthe pixels would appear to be half on. In the case in which the image isinverted at a frequency slower than the critical clicker rate of thehuman eye, the viewer would see the image switching between the positiveimage and the inverted image. Neither of these situations would providea usable display.

In one approach to solving this problem, the light source used toilluminated the SLM is switched off or directed away from the SLM duringthe time when the frame is inverted. This type of system is described incopending U.S. patent application Ser. No. 08-361,775, filed Dec. 22,1994, entitled DC FIELD-BALANCING TECHNIQUE FOR AN ACTIVE MATRIX LIQUIDCRYSTAL IMAGE GENERATOR, which is incorporated herein by reference.However, this approach substantially limits the brightness andefficiency of the system. In the case where the magnitude of theelectric field during the DC field-balancing and the time when the frameis inverted is equal to the magnitude of the electric field and the timewhen the frame is viewed, only a maximum of 50% of the light from agiven light source may be utilized. This is illustrated in FIG. 1a whichis a timing diagram showing the relationship between the switching onand off of the light source and the switching of the SLM image data.

As shown in FIG. 1a, the light source is switched on for a period oftime indicated by T1. During this time T1, the SLM is switched to form adesired image. In order to DC balance the SLM, the SLM is switched toform the inverse of the desired image during a time period T2. In orderto prevent this inverse image from distorting the desired image, thelight source is switched off during the time T2 as shown in FIG. 1a.

In order to establish a convention to be used throughout thisdescription, the operation of a given pixel 10 of a reflective type FLCSLM using the above mentioned approach of switching off the light sourceduring the time the frame is inverted will be described with referenceto FIGS. 1b-d. FIG. 1b shows pixel 10 when it is in its bright state andFIG. 1c shows pixel 10 when it is in its dark state. As illustrated inboth FIGS. 1b and 1c, a light source 12 directs light, indicated byarrow 14, into a polarizer 16. Polarizer 16 is arranged to allow, forexample, horizontally linearly polarized light, indicated by thereference letter 11 and by arrow 18, to pass through polarizer 16.However, polarizer 16 blocks any vertically linearly polarized componentof the light and thereby directs only horizontally linearly polarizedlight into pixel 10. This arrangement insures that only horizontallylinearly polarized light is used to illuminate pixel 10. For purposes ofclarity throughout this description, the various configurations will bedescribed using horizontally linearly polarized light as the initialinput light for each of the various configurations.

As also illustrated in FIGS. 1b and 1c, pixel 10 includes a reflectivebackplane 22 and a layer of FLC material 24 which is supported in frontof reflective backplane 22 and which acts as the light modulatingmedium. The various components would typically be positioned adjacentone another, however, for illustrative purposes, the spacing between thevarious components is provided. In this example, the FLC material has athickness and a birefringence which cause the material to act as aquarter wave plate for a given wavelength. In this example, the FLCmaterial is typical of those readily available and has a birefringenceof 0.142. Therefore a thickness of 900 nm causes the SLM to act as aquarter wave plate for a wavelength of approximately 510 nm.

FLC material 22 has accompanying alignment layers (not shown) at thesurfaces which have a buff axis or alignment axis that controls thealignment of the molecules of the FLC material. For this example of areflective mode SLM, the SLM is oriented such that the alignment axis isrotated 22.5 degrees relative to the polarization of the horizontallylinearly polarized light being directed into the SLM. The FLC also has atilt angle of 22.5 degrees associated with the average optic axis of themolecules making up the FLC material. Therefore, when FLC material 24 ofthe pixel is switched to its first state, in this case by applying a +5VDC electric field across the pixel, the optic axis is rotated to a 45degree angle relative to the horizontally linearly polarized light. Thiscauses the pixel to act as a quarter wave plate for horizontallylinearly polarized light at 510 nm. Alternatively, when the pixel isswitched to its second state, in this case by applying a -5 VDC electricfield across the pixel, the optic axis is rotated to a zero degree anglerelative to the horizontally linearly polarized light. This causes thepixel to have no effect on the horizontally linearly polarized lightdirected into the pixel. In other words, the tilt angle is the anglethat the FLC optic axis is rotated one side or the other of the buffaxis when the FLC material is switched to its first and second states.

Now that the configuration of the pixel for this example has beendescribed, its effect on the light as it passes through the variouselements will be described. Initially, it will be assumed the light ismonochrome at the wavelength at which the SLM acts as a quarter waveplate, in this case 510 nm. As illustrated in FIG. 1b, when the FLCmaterial is switched to its first state, which will be referred tohereinafter as its A state, FLC material 24 converts the 510 nmwavelength horizontally linearly polarized light directed into the pixeland indicated by arrow 18 into circularly polarized light indicated bythe reference letters C and arrow 26. Reflective backplane 22 reflectsthis circularly polarized light as indicated by arrow 28 and directingit back into FLC material 24. FLC material 24 again acts on the lightconverting it from circularly polarized light to vertically linearlypolarized light as indicated by reference letter V and arrow 30. Thevertically linearly polarized light 30 is directed into an analyzer 32which is configured to pass vertically linearly polarized light andblock horizontally polarized light. Since analyzer 32 is arranged topass vertically linearly polarized light, this vertically linearlypolarized light indicated by arrow 30 passes through analyzer 32 to aviewing area indicated by viewer 34 causing the pixel to appear brightto the viewer.

Alternatively, as illustrated in FIG. 1c, FLC material 24 has no effecton the horizontally linearly polarized light directed into the pixelwhen the pixel is in its second state, which will be referred tohereinafter as its B state. This is the case regardless of thewavelength of the light. Therefore, the horizontally linearly polarizedlight passes through FLC material 24 and is reflected by reflectivebackplane 22 back into FLC material 24. Again, FLC material 24 has noeffect on the horizontally linearly polarized light. And finally, sinceanalyzer 32 is arranged to block horizontally linearly polarized light,the horizontally linearly polarized light is prevented from passingthrough to viewing area 34 causing the pixel to appear dark.

Although the polarization state of the light is relatively straightforward when the light is assumed to be at a wavelength at which the SLMacts as a quarter wave plate, it becomes more complicated whenpolychromatic light is used. This is because even if the birefringenceΔn of the FLC were constant, the retardance of the SLM in waves wouldvary with wavelength; furthermore, the birefringence of the FLC materialalso varies as the wavelength of the light varies. In displayapplications, this becomes very important due to the desire to providecolor displays. FIG. 1d illustrates the effects the SLM has on visiblelight ranging in wavelength from 400 nm to 700 nm as a function of thewavelength of the light assuming typical FLC birefringence dispersions.Solid line 36 corresponds to the first case when the pixel is in its Astate as illustrated in FIG. 1b and the dashed line 38 corresponds tothe second case when the pixel is in its B state as illustrated in FIG.1c. As is illustrated in FIG. 1d, the resulting output of thisconfiguration varies substantially depending on the wavelength of thelight as indicated by line 36. In fact, only a little more than 50% ofthe horizontally linearly polarized light at 400 nm that is directedinto the SLM is converted to vertically linearly polarized light usingthis configuration.

The above described configuration makes use of crossed polarizers. Thatis, polarizer 16 blocks vertically linearly polarized light and analyzer32 blocks horizontally linearly polarized light. This means thatpolarizer 16 and analyzer 32 must be different elements. If bothpolarizer 16 and analyzer 32 were configured to pass the samepolarization of light, they would be referred to as parallel polarizersand could be provided by the same element.

In an alternative system configuration, a polarizing beam splitter maybe used to replace both the polarizer and the analyzer. FIGS. 1e and 1fillustrate such a system when pixel 10 is in its A and B statesrespectively. In this alternative system, light from light source 12 isdirected into a polarizing beam splitter (PBS) 40 as indicated by arrow42. PBS 40 is configured to reflect horizontally linearly polarizedlight as indicated by arrow 44 and pass vertically linearly polarizedlight as indicated by arrow 46. The horizontally linearly polarizedlight indicated by arrow 44 is directed into SLM 24.

When pixel 10 is in its A state as illustrated in FIG. 1e, SLM 24 actsas a quarter wave plate as described above converting the horizontallylinearly polarized light to circularly polarized light and reflectivebackplane 22 reflects this light back into SLM 24. Again, SLM 24converts this circularly polarized light into vertically linearlypolarized light as described above for FIG. 1b and as indicated by arrow48. Since PBS 40 is configured to pass vertically linearly polarizedlight, this light passes through PBS 40 into viewing area 34 causingpixel 10 to appear bright.

When pixel 10 is in its B state as illustrated in FIG. 1f, SLM 24 has noeffect on the horizontally linearly polarized light. Therefore, thehorizontally linearly polarized light that is directed into SLM 24 asindicated by arrow 44 remains horizontally linearly polarized light asit passes through SLM 24, is reflected by backplane 22, and again passesthrough SLM 24. However, since PBS 40 is configured to reflecthorizontally linearly polarized light, this light is reflected backtoward light source 12 as indicated by arrow 50 causing pixel 10 toappear dark.

As mentioned above, in the configuration currently being described, thelight source is turned off during the time in which the image isinverted for purposes of DC field-balancing the FLC material asillustrated in FIG 1a. This substantially reduces the brightness orefficiency of the display. In order to overcome this problem of notbeing able to view the system during the DC field-balancing frameinversion time, compensator cells have been proposed for transmissiveSLMs such as those described in U.S. Pat. No. 5,126,864. Thesecompensator cells are intended to correct for the frame inversion duringthe time when the FLC pixel is being operated in its inverted state.FIG. 2a illustrates a transmissive mode system 200 which includes an SLM202, a compensator cell 204, a polarizer 206, and an analyzer 208.

As described above for the FLC material of the SLM of the previousconfiguration, SLM 202 and compensator cell 204 each include an FLClayer which is switchable between an A and a B state. This results infour possible combinations of states for the SLM and compensator cell.For purposes of consistency in comparing various configurationsdescribed herein, these four cases will be defined as follows:

Case 1--compensator cell in B state, SLM pixel in A state

Case 2--compensator cell in B state, SLM pixel in B state

Case 3--compensator cell in A state, SLM pixel in B state

Case 4--compensator cell in A state, SLM pixel in A state

For this configuration, Cases 1 and 2 correspond to the normal operationof the system during which the compensator cell is in its B state andthe SLM pixels are switched between their A and B states to respectivelyproduce a bright or dark pixel. This is illustrated in the first half ofFIG. 2b which is a timing diagram showing the states of the lightsource, the SLM, and the compensator cell. As shown in FIG. 2b, thelight source remains ON throughout the operation of the system. Duringthe first half of the time illustrated in FIG. 2b, the pixels of the SLMare switched between their A and B states to produce a desired image.Cases 3 and 4 correspond to the time during which the frame is invertedfor purposes of DC field balancing (i.e. the SLM pixel states must bereversed) and the compensator cell is switched to its A state tocompensate for the inversion. This is illustrated by the second half ofthe diagram of FIG. 2b. To properly DC field-balance the display as wellas allow the display to be viewed continuously, Case 1 and Case 3 mustgive the same results and Case 2 and Case 4 must give the same results.That is, for this configuration, Cases 1 and 3 must both produce abright pixel and Cases 2 and 4 must both produce a dark pixel.

In this example of a transmissive mode system, both the FLC layer of theSLM pixel and the compensator cell are 1800 nm thick which causes themto act as a half wave plate for a wavelength of 510 nm when in the ONstate. In this configuration, the polarizer and analyzer perform thefunctions performed by polarizer 16 and analyzer 32, or alternativelyPBS 40, of the reflective mode systems described above. Polarizer 206 ispositioned optically in front of compensator cell 204 and the SLM pixel202 such that it allows only horizontally linearly polarized light topass through it into compensator cell 204. Also, analyzer 208 which onlyallows vertically linearly polarized light to pass through is positionedoptically behind SLM 202.

FIGS. 2c and 2d illustrate the net result the above describedtransmissive system configuration has on light directed in to thesystem. FIG. 2c shows the results for Case 1 and 2 during which thecompensator cell is in its B state and the SLM is switched between its Astate for Case 1 and its B state for Case 2. Case 1 is indicated bysolid line 210 and Case 2 is indicated by dashed line 212. FIG. 2d showsthe results for Case 3 and 4 during which the compensator cell is in itsA state and the SLM is switched between its B state for Case 3 and its Astate for Case 4. Case 3 is represented by solid line 214 and Case 4 isrepresented by dashed line 216.

As clearly shown by FIGS. 2c and 2d, this transmissive configurationproduces identical results, that is a bright pixel, for Case 1 and 3 asindicated by lines 210 and 214, respectively. It also produces identicalresults for Cases 2 and 4 as indicated by lines 212 and 216,respectively. It should also be noted that this configuration producesrelatively good results over the entire wavelength range from 400 nm to700 nm. The worst results are at 400 nm where approximately 80% of thehorizontally linearly polarized light is converted to verticallypolarized light.

Although the compensator cell approach works well for a transmissive SLMas described above, applicant has found that this same general approachdoes not work as well for a reflective type SLM. To illustrate thisdifference, and referring to FIG. 3a, a reflective type display system300 including a reflective type SLM 302 having a reflective backplane303, a compensator cell 304, a polarizer 306, and an analyzer 308 willbe described. Compensator cell 304 is positioned adjacent to SLM 302. Asdescribed above for FIGS. 1b and 1c, polarizer 306 is positioned todirect only horizontally linearly polarized light into compensator cell304. Because the light passes through the SLM and the compensator celltwice in a reflective mode system, the FLC material of SLM 302 andcompensator cell 304 are configured to act as quarter wave plates for awavelength of 510 nm rather than half wave plates as described above forthe transmissive system of FIG. 2a.

In this example, the FLC materials of both SLM 302 and compensator cell304 are 900 nm thick and both have a tilt angle of 22.5 degrees. Thebuff axis of the SLM is aligned with the horizontally linearly polarizedlight directed into the system by polarizer 306. Also, the buff axis ofcompensator cell 304 is positioned perpendicular to the buff axis of SLM302. FIGS. 3b and 3c illustrate the net result that system 300 has onlight directed in to the system. FIG. 3b shows the results for Case 1and 2 during which the compensator cell is in its B state and the SLM isswitched between its A state for Case 1 and its B state for Case 2. Case1 is indicated by solid line 310 and Case 2 is indicated by dashed line312. FIG. 3c shows the results for Case 3 and 4 during which thecompensator cell is in its A state and the SLM is switched between its Bstate for Case 3 and its A state for Case 4. Case 3 is represented bysolid line 314 and Case 4 is represented by dashed line 316.

As clearly shown by FIGS. 3b and 3c, system 300 produces identicalresults, that is, a bright pixel for Case 1 and 3 as indicated by lines310 and 314, respectively. It also produces identical results for Cases2 and 4 as indicated by lines 312 and 316, respectively. However, thisconfiguration does not produces very good results over the entirewavelength range from 400 nm to 700 nm. The worst results are at 400 nmwhere only approximately 5% of the horizontally linearly polarized lightis converted to vertically polarized light. At a wavelength of about 500nm about 50% of the horizontally linearly polarized light is convertedto vertically linearly polarized light. The best results are at 700 nmwhere about 80% of the horizontally linearly polarized light isconverted to vertically linearly polarized light. Since the point toadding the compensator cell is to increase the efficiency or brightnessof the system, this arrangement does not improve the efficiency orbrightness for the lower wavelength range when compared to the system ofFIG. 1b and 1c which simply turns OFF the light source during the DCfield-balancing time.

As can be clearly seen when comparing FIGS. 3b-c to FIGS. 2c-d, theeffects on the light caused by the various components of the reflectiveconfiguration of FIG. 3a are very much different from the effects on thelight caused by the transmissive configuration of FIG. 2a. That is, thereflective configuration of FIG. 3a is not optically equivalent to thetransmissive configuration of FIG. 2a even though it may initially seemas though they should be optically equivalent. These two configurationsare optically different from one another because the light must passthrough the SLM and compensator cell twice in the reflectiveconfiguration with the first pass through the compensator being beforethe two passes through the SLM and the second pass through thecompensator cell being after the two passes through the SLM.

Due to this difference in the transmissive and reflectiveconfigurations, it has proved difficult to provide a reflective typesystem which is DC field-balanced and is substantially continuouslyviewable while providing improved efficiency or brightness compared to asystem which simply turns off the light source during the DCfield-balancing portion of the frame. The present invention providesarrangements and methods for overcome this problem.

SUMMARY OF THE INVENTION

As will be described in more detail hereinafter, a reflection mode,spatial light modulating system and methods of operating the system areherein disclosed. The reflection mode, ferroelectric liquid crystalspatial light modulating system, includes a light reflecting typespatial light modulator. The spatial light modulator has a lightreflecting surface cooperating with a layer of ferroelectric liquidcrystal light modulating medium switchable between first and secondstates so as to act on light in different first and second ways,respectively. A switching arrangement switches the liquid crystal lightmodulating medium between the first and second states and anillumination arrangement produces a source of light. An opticsarrangement is optically coupled the spatial light modulator and theillumination arrangement such that light is directed from the source oflight into the spatial light modulator for reflection back out of themodulator and such that reflected light is directed from the spatiallight modulator into a predetermined viewing area. A compensator cell isalso positioned in the optical path between the light source and theviewing area. The compensator cell has a layer of ferroelectric liquidcrystal light modulating medium switchable between a primary and asecondary state so as to act on light in different primary and secondaryways, respectively.

In one embodiment, the optics arrangement includes a passive quarterwave plate positioned in the optical path between the light source andthe spatial light modulator and in the optical path between the spatiallight modulator and the viewing area. In this embodiment, thecompensator cell is positioned in the optical path between the lightsource and the spatial light modulator and in the optical path betweenthe spatial light modulator and the viewing area.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention may best be understood byreference to the following description of the presently preferredembodiments together with the accompanying drawings.

FIG. 1a is a timing diagram illustrating the timing at which a lightsource for a prior art DC-balanced display system is switched ON andOFF.

FIGS. 1b and 1c are diagrammatic cross sectional views of a pixel of aprior art reflective type SLM display system illustrating how the pixelacts on light when the pixel is in the ON and OFF states.

FIG. 1d is a graph illustrating the effects the system of FIG. 1b and 1chas on light after it passes through the system.

FIGS. 1e and 1f are diagrammatic cross sectional views of a pixel of aprior art reflective type SLM display system including a polarizing beamsplitter.

FIG. 2a is a diagrammatic cross sectional view of a prior arttransmissive SLM display system.

FIG. 2b is a timing diagram illustrating the timing at which a lightsource for a prior art DC-balanced display system is switched ON andOFF.

FIGS. 2c and 2d are graphs illustrating the effects the system of FIG.2a has on light after it passes through the system.

FIG. 3a is a diagrammatic cross sectional view of a prior art reflectiveSLM display system.

FIGS. 3b and 3c are graphs illustrating the effects the system of FIG.3a has on light after it passes through the system.

FIG. 4a is a diagrammatic cross sectional view of a first embodiment ofa reflective SLM display system designed in accordance with the presentinvention.

FIGS. 4b-c are graphs illustrating the effects the system of FIG. 4a hason light after it passes through the system.

FIG. 5a is a diagrammatic cross sectional view of a second embodiment ofa reflective SLM display system designed in accordance with the presentinvention.

FIGS. 5b-c are graphs illustrating the effects the system of FIG. 5a hason light after it passes through the system.

FIG. 5 is a diagrammatic cross sectional view of a third embodiment of areflective SLM display system designed in accordance with the presentinvention.

FIGS. 7a-b are diagrammatic cross sectional views of a fourth embodimentof a reflective SLM display system designed in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An invention is described for providing methods and apparatus forproducing a substantially continuously viewable reflective type SLMdisplay system which is DC field-balanced and which is more efficient orbrighter than would be possible using a reflective type SLM displaysystem which simply turns off the light source during the DC fieldbalancing portion of each image frame. In the following description,numerous specific details are set forth in order to provide a thoroughunderstanding of the present invention. However, based on the followingdescription, it will be obvious to one skilled in the art that thepresent invention may be embodied in a wide variety of specificconfigurations. Also, well known processes for producing variouscomponents and certain well known optical effects of various opticalcomponents will not be described in detail in order not to unnecessarilyobscure the present invention.

Referring initially to FIG. 4a, the present invention will be describedwith reference to a first embodiment of the invention which takes theform of a reflective type SLM display system generally designated byreference numeral 400. As illustrated in FIG. 4a, system 400 includes anSLM 402 having a reflective backplane 403, a compensator cell 404, apolarizer 405, and an analyzer 406. Alternatively, in the same manner asdescribed above, crossed polarizer 405 and analyzer 406 may be replacedwith a polarizing beam splitter.

System 400 is configured in a manner similar to that described above forsystem 300 of FIG. 3a. That is, compensator cell 404 is positionedadjacent SLM 402. Also, polarizer 405 is positioned to direct onlyhorizontally linearly polarized light into compensator cell 404.Similarly, analyzer 406 allows only vertically linearly polarized lightto pass through it and into the viewing area after the light directed into the system has passed through compensator cell 404 and SLM 402 andbeen reflected back through SLM 402 and compensator cell 404. However,in accordance with the invention, system 400 also includes a staticquarter wave plate 408 positioned optically between compensator cell 404and polarizer 405 and analyzer 406.

As would be understood by those skilled in the art, SLM 402 may be madeup of an array of any number of individually controllable pixels whichare individually switchable between two states. For purposes ofconsistency, it will be assumed that each pixel is switched to its Astate by applying a +5 VDC electric field through the pixel and eachpixel is switched to its B state by applying a -5 VDC electric fieldthrough the pixel. It should be understood that the present invention isnot limited to these specific voltages and would equally applyregardless of the voltages used to switch the pixels.

System 400 further includes a light source 410 for directing light intothe system in a manner similar to that described above for FIGS. 1b and1c. With this configuration, light source 410 directs light intopolarizer 405 as indicated by arrow 412. Polarizer 405 blocks anyvertically linearly polarized portions of the light from passing throughpolarizer 405 an allows only horizontally linearly polarized portions ofthe light to pass through polarizer 405 into static quarter wave plate408. This light passes through static quarter wave plate 408,compensator cell 404, and SLM 402 and is then reflected by reflectivebackplane 403 back through SLM 402, compensator cell 404, and staticwave plate 408 to analyzer 406 as illustrated in FIG. 4a. Analyzer 406then blocks any horizontally linearly polarized portions of the lightand allows only vertically linearly polarized portions of the light topass through it to a viewing area indicated by viewer 416. Sincepolarizer 405 blocks vertically linearly polarized light and analyzer406 blocks horizontally linearly polarized light, this type of system isreferred to as using crossed polarizers.

For this embodiment and as described above for system 300, because thelight passes through the SLM and the compensator cell twice in areflective mode system, the FLC material of SLM 402 and compensator cell404 are configured to act as quarter wave plates for a wavelength of 510nm. In this configuration, the FLC materials of both SLM 402 andcompensator cell 404 are 900 nm thick and both have a tilt angle of 22.5degrees. In this specific embodiment, the buff axis of the SLM ispositioned at a 22.5 degree angle relative to the horizontally linearlypolarized light directed into the system. Also, for this embodiment, thebuff axis of compensator cell 404 is positioned perpendicular to thebuff axis of SLM 402.

Although the buff axis of the SLM is described as being positioned at22.5 degrees relative to the horizontally linearly polarized lightdirected into the system, this is not a requirement. In fact, thisconfiguration works equally as well regardless of the orientation of theSLM buff axis relative to the horizontally linearly polarized lightdirected into the system so long as the buff axis of the compensatorcell is oriented perpendicular to the buff axis of the SLM. This freedomin orienting the buff axis of the SLM relative to the horizontallylinearly polarized light directed into the system makes this overallsystem easier to produce than other conventional systems because onlythe orientation of the SLM relative to the compensator cell must beprecisely controlled.

The orientation of the static quarter wave plate relative to thehorizontally linearly polarized light directed into the system is alsoimportant. Generally, static quarter wave plate 408 has a primary axiswhich is oriented at a 45 degree angle to the horizontally linearlypolarized light directed into the quarter wave plate.

Although the tilt angles of SLM 402 and compensator cell 404 aredescribed as being 22.5 degrees, this is not a requirement. Theconfiguration described above for this embodiment works regardless ofthe tilt angle of the FLC material of the SLM and the compensator cell,but works best when the tilt angles of the two components are the same.Therefore, it should be understood that the present invention wouldequally apply to systems using SLMs and compensator cells having tiltangles other than 22.5 degrees. With this configuration, the brightstates obtained by the system remain bright regardless of the tilt angleused provided the tilt angles match. However, the use of tilt angles inthe range of 22.5 to 25.5 degrees provides optimum dark stateextinction, with the choice of tilt angle at the low end of the rangeproviding best extinction over a narrow range of wavelengths centered onthe wavelength for which the SLM and compensator have quarter-waveretardance and with the choice of tilt angle towards the tipper end ofthe range providing good extinction over a more extended range ofwavelength. Increasing the tilt angle past 25.5 degrees eventuallyreduces dark state extinction.

Now that the physical configuration of system 400 has been described,its effect on light directed into system 400 will be described. FIGS. 4band 4c illustrate the net result that system 400 has on light directedin to the system. FIG. 4b shows the results for Case 1 and 2 duringwhich the compensator cell is in its B state and the SLM is switchedbetween the A state for Case 1 and the B state for Case 2. Case 1 isindicated by solid line 420 and Case 2 is indicated by dashed line 422.FIG. 4c shows the results for Case 3 and 4 during which the compensatorcell is in its A state and the SLM is switched between the B state forCase 3 and the A state for Case 4. Case 3 is represented by solid line424 and Case 4 is represented by dashed line 426. Cases 1-4 correspondto Cases 1-4 for the systems described above in the background.

As illustrated in FIGS. 4b and 4c, because of quarter wave plate 408 isincluded in the configuration of system 400, Cases 1 and 3 result in adark pixel rather than a bright pixel and Cases 2 and 4 result in abright pixel rather than a dark pixel. This is the opposite of theresults described in the background. However, this inversion of thebright and the dark states may be compensated for in a variety of wayssuch as reversing the A and the B states for the SLM (i.e. using a -5VDC to switch the pixel to the A state and using a 5 VDC to switch thepixel to the B state). The important thing is that the results of Cases1 and 3 are identical and the results of Cases 2 and 4 are identical.

For system 400, static quarter wave plate 408 is preferably a readilyprovidable achromatic quarter wave plate. The use of an achromaticstatic quarter wave plate provides the best results over a broad colorspectrum because it flattens out the curves 422 of FIG. 4b and 426 ofFIG. 4c representing the bright states obtained by Case 1 and Case 2.This flattening out of the curve improves the optical throughput ofsystem 400 by increasing the amount of light which passes through thesystem for a given pixel when the combination of that pixel and theother elements are switched to produce a bright state.

In one embodiment of the invention which reverses the bright and darkstates described above for FIGS. 4a-c, parallel polarizers are usedinstead of crossed polarizers. FIG. 5a-c illustrate a system 500 whichutilizes parallel polarizers. As described above for system 400, system500 includes a SLM 502, a reflective backplane 503, a compensator cell504, a polarizer 505, a static quarter wave plate 508, and a lightsource 510. Light source 510 directs light into polarizer 505 whichblocks any vertically linearly polarized light and allows onlyhorizontally linearly polarized light to pass through. This horizontallylinearly polarized light then passes through and is acted upon by staticquarter wave plate 508, compensator cell 504, SLM 502, and reflectivebackplane 503 in the same way as described above for FIG. 4a. However,in this embodiment, polarizer 505 also acts as the analyzer for thesystem. This use of polarizer 505 for both the polarizer and theanalyzer is what makes this system a parallel polarizer system.

In the configuration of FIG. 5a, polarizer 505 acts as the analyzer byblocking any vertically linearly polarized light and allowing anyhorizontally linearly polarized light to pass into the viewing area.This is the opposite of the polarizations of light blocked and passed byanalyzer 406 in system 400. This has the effect of reversing the brightand dark states of the system and results in the net effects illustratedin FIGS. 5b and 5c. FIG. 5b shows the results for Case 1 and 2 duringwhich the compensator cell is in its B state and the SLM is switchedbetween the A state for Case 1 and the B state for Case 2. Case 1 isindicated by solid line 520 and Case 2 is indicated by dashed line 522.FIG. 5c shows the results for Case 3 and 4 during which the compensatorcell is in its A state and the SLM is switched between the B state forCase 3 and the A state for Case 4. Case 3 is represented by solid line524 and Case 4 is represented by dashed line 526. Cases 1-4 correspondto Cases 1-4 for the systems described above in the background and Cases1-4 described above for FIG. 4.

As clearly shown by FIGS. 5b and 5c, system 500 produces identicalresults, that is, a bright pixel for Case 1 and 3 as indicated by lines520 and 524, respectively. It also produces identical results for Cases2 and 4 as indicated by lines 522 and 526, respectively. Thisconfiguration also produces very good results over the entire wavelengthrange from 400 nm to 700 nm. In fact, as illustrated by lines 522 and526, this configuration provides substantially uniform blockage of theentire range of wavelengths of the light that is directed into thespatial light modulator. Also, in both Cases 1 and 3, a large portion ofthe horizontally linearly polarized light passes through the system forthe entire range of 400 nm to 700 nm. Since the point to adding thecompensator cell is to increase the efficiency or brightness of thesystem, this arrangement dramatically improves the efficiency orbrightness of system 500 over the complete wavelength range whencompared to the system of FIG. 1b and 1c which simply turns OFF thelight source during the DC field-balancing time. This also substantiallyimproves the efficiency of the system compared to system 300 of FIG. 3described above which does not include the static quarter wave plate.Furthermore, since essentially no light from the light source passesthrough the system to the viewing area when the elements are switched toproduce a dark state as indicated by lines 522 and 526, thisconfiguration also provides an excellent contrast ratio.

In another embodiment similar to system 400 of FIG. 4a, a birefringentelement may be added to system 400 in order to provide results verysimilar to the results obtained by system 500 of FIG. 5a. Using likereference numerals to represent like components, FIG. 6 illustrates asystem 600 including SLM 402, reflective backplane 403, compensator cell404, polarizer 405, analyzer 406, static quarter wave plate 408, andlight source 410. As described above for FIG. 4, polarizer 405 andanalyzer 406 are crossed polarizers. However, in accordance with thisembodiment of the invention, system 600 further includes an additionalbirefringent element 612 which can be positioned between SLM 402 andcompensator cell 404, as shown here, or alternately, can be positionedbetween compensator cell 404 and static quarter wave plate 408.

In this embodiment, birefringent element 612 is a commercially availablepolycarbonate film having a retardance of approximately one half of thewavelength of the light for which the system is optimized, for example awavelength of 510 nm. Alternatively, birefringent element 612 may be anybirefringent material capable of providing the desired retardance suchas poly vinyl alcohol or any other optically clear birefringentmaterial.

In this embodiment, the buff axes of SLM 402 and compensator cell 404are parallel to one another and birefringent element 612 has a primaryaxis which is oriented perpendicular to the buff axis of both SLM 402and compensator cell 404. As describe above for system 400, polarizer405 directs horizontally linearly polarized light into quarter waveplate 408 and quarter wave plate 408 is oriented at a 45 degree angle tothe horizontally linearly polarized light. SLM 402, compensator cell404, and birefringent element 612 may be oriented in any way relative toquarter wave plate 408 so long as the buff axes of SLM 402 andCompensator cell 404 are parallel to one another and the primary axis ofbirefringent element 612 is perpendicular to the buff axes of SLM 402and compensator cell 404.

The addition of the birefringent element causes Case 1 and Case 3 forthis embodiment to result in a bright state in which the throughputvaries only slightly over the range of the wavelengths similar to curves520 and 524 of FIGS. 5b and 5c. Also, the addition of the birefringentelement causes Case 2 and Case 4 for this embodiment to result in asubstantially more uniform dark state similar to lines 522 and 526 ofFIGS. 5b and 5c. This results in a system that is able to provide a highcontrast ratio while maintaining a relatively high throughput for theentire wavelength range even though crossed polarizers are utilized.

Although the above described embodiments have been described as havingthe static quarter wave plate positioned between the polarizer and thecompensator cell, this is not a requirement. Instead, the static quarterwave plate may be located between the compensator cell and SLM and stillremain within the scope of the invention.

In another embodiment, an off axis system may be utilized in order toprovide a continuously viewable DC field-balanced reflective displaysystem. FIGS. 7a and 7b illustrate one embodiment of an off axis displaysystem 700. As illustrated in FIGS. 7a and 7b, system 700 includes a SLM702, a reflective backplane 703, a compensator cell 704, a polarizer705, an analyzer 706, and a light source 710. In this embodiment, thelight is directed into the SLM at an angle and reflected back into aviewing area indicated by viewer 720 such that the light directed intothe system only passes through the compensator cell once rather thanpassing through the compensator cell twice as described above for thepreviously described embodiments.

Since the light only passes through compensator cell 704 once, thethickness of compensator cell 704 is configured to be twice thethickness of the SLM. Generally, SLM 702 has a thickness which causesSLM 702 to act as a quarter wave plate when switched to its A state andcompensator cell 704 has a thickness which causes it to act as a halfwave plate when it is switched to its A state. Therefore, in the case inwhich an FLC material is used for both the SLM and compensator cell thathas a birefringence of 0.142, the thickness FLC material for the SLMwould be approximately 900 nm and the thickness of the FLC material forthe compensator cell would be approximately 1800 nm. Both SLM 702 andcompensator cell are configured to have substantially no effect on thepolarization of the light passing through them when they are switched totheir B states.

For the configuration being described, polarizer 705 is configured toallow only horizontally linearly polarized light to be directed into thesystem. Analyzer 706 is configured to allow only vertically linearlypolarized light to pass into the viewing area. Also, for thisembodiment, the buff axis of compensator cell 704 is orientedperpendicular to the buff axis of SLM 702 and the buff axis of SLM 702is advantageously oriented parallel to horizontally linearly polarizedlight directed into the system. Other orientations of the buff axes arealso effective provided that the SLM and compensator cell buff axesremain perpendicular to one another.

As described above for the previous embodiments, the off axisconfiguration shown in FIGS. 7a and 7b provide identical results forCases 1 and 3 and Cases 2 and 4. This configuration also provides goodresults over a broad spectrum similar to the results illustrated inFIGS. 5b and 5c. Therefore, system 700 is also able to provide acontinuously viewable system which more effectively utilizes light fromthe light source when compared to the conventional reflective systemsillustrated in FIGS. 1b-c and FIG. 3a.

Although only certain specific embodiments of the present invention havebeen described in detail, it should be understood that the presentinvention may be embodied in many other specific forms without departingfrom the spirit or scope of the invention. For example, although thesystems have been described above as using horizontally linearlypolarized light as the initial input light polarization, this is not arequirement. Instead, it should be understood that the initial inputlight polarization may alternatively be vertically linearly polarizedlight. Therefore, the present examples are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope of theappended claims.

What is claimed is:
 1. A reflection mode, ferroelectric liquid crystalspatial light modulating system, comprising:(a) a light reflecting typespatial light modulator including a light reflecting surface cooperatingwith a layer of ferroelectric liquid crystal light modulating mediumswitchable between first and second states so as to act on light indifferent first and second ways, respectively; (b) a switchingarrangement for switching the liquid crystal light modulating mediumbetween the first and second states; (c) an illumination arrangement forproducing a source of light; and (d) an optics arrangement opticallycoupled to the spatial light modulator and the illumination arrangementfor directing light along a first optical path from the source of lightinto the spatial light modulator for reflection back out of saidmodulator and for directing reflected light along a second optical pathfrom the spatial light modulator into a predetermined viewing area, theoptics arrangement including(i) a passive wave plate positioned in boththe first optical path between the light source and the spatial lightmodulator and in the second optical path between the spatial lightmodulator and the viewing area; and (ii) a compensator cell positionedin both the first optical path between the light source and the spatiallight modulator and in the second optical path between the spatial lightmodulator and the viewing area, the compensator cell having a layer offerroelectric liquid crystal light modulating medium switchable betweena primary and a secondary state so as to act on light in differentprimary and secondary ways, respectively.
 2. A system according to claim1 wherein the passive wave plate is a quarter wave plate.
 3. A systemaccording to claim 1 wherein:(a) the spatial light modulator is anactive matrix spatial light modulator with its liquid crystal lightmodulating medium being divided into an array of individuallycontrollable pixels; and (b) the switching means includes means forswitching each of the pixels between the first and second states.
 4. Asystem according to claim 1 wherein the optics arrangement furtherincludes a polarizer element positioned in the first optical pathbetween the light source and the spatial light modulator for polarizingthe light directed in to the spatial light modulator and an analyzerelement positioned in the second optical path between the spatial lightmodulator and the viewing area for allowing light of a certainpolarization to pass through the analyzer to the viewing area.
 5. Asystem according to claim 4 wherein the polarizer and analyzer arecrossed to one another.
 6. A system according to claim 4 wherein thepolarizer and analyzer are oriented parallel to one another.
 7. A systemaccording to claim 6 wherein the polarizer and analyzer are provided bya single polarizing element positioned in both the first optical pathbetween the light source and the spatial light modulator and in thesecond optical path between the spatial light modulator and the viewingarea.
 8. A system according to claim 1 wherein:(a) the opticsarrangement further includes a combination polarizer-beamsplitter-analyzer positioned in both the first optical path between thesource of light and the spatial light modulator and in the secondoptical path between the spatial light modulator and the viewing area soas to be able to direct light from the source of light to the spatiallight modulator and from the spatial light modulator to the viewingarea; (b) the passive wave plate is positioned optically between thespatial light modulator and the combination polarizer-beamsplitter-analyzer; and (c) the compensator cell is positioned opticallybetween the spatial light modulator and the combination polarizer-beamsplitter-analyzer.
 9. A system according to claim 8 wherein thecompensator cell is positioned optically between the spatial lightmodulator and the passive quarter wave plate.
 10. A system according toclaim 1 wherein the ferroelectric liquid crystal of both the spatiallight modulator and the compensator cell each has a buff axis, the buffaxis of the spatial light modulator being oriented perpendicular to thebuff axis of the compensator cell.
 11. A system according to claim 1wherein the ferroelectric liquid crystal layers of both the spatiallight modulator and the compensator cell have tilt angles andretardances that are equal to one another.
 12. A system according toclaim 1 wherein the ferroelectric liquid crystal layers of both thespatial light modulator and the compensator cell have the samethickness.
 13. A system according to claim 12 wherein the spatial lightmodulator and the compensator cell both act as quarter wave plates. 14.An optics arrangement for directing light from a light source along afirst optical path into a reflective type liquid crystal spatial lightmodulator that is switchable between a first and a second state and fordirecting light reflected from the spatial light modulator along asecond optical path into a predetermined viewing area, the opticsarrangement comprising:parallel polarizers for polarizing the lightdirected into the spatial light modulator and for analyzing the lightdirected from the spatial light modulator to the viewing area, theoptics arrangement being configured such that the parallel polarizersprovide substantially uniform blockage of the light directed from thespatial light modulator into the viewing area when the spatial lightmodulator is in the second state, the blockage of the light beingsubstantially independent of wavelength, and a passive wave platelocated in both the first optical path from the light source to thespatial light modulator and in the second optical path from the spatiallight modulator to the viewing area.
 15. A system according to claim 14wherein the parallel polarizers are provided by a single polarizerlocated in both the first optical path from the light source to thespatial light modulator and in the second optical path from the spatiallight modulator to the viewing area.
 16. A system according to claim 14wherein the spatial light modulator is an active matrix spatial lightmodulator divided into an array of individually controllable pixels eachof which is switchable between an On and an Off state.
 17. A systemaccording to claim 14 wherein the passive wave plate is a quarter waveplate.
 18. A system according to claim 17 wherein the optics arrangementfurther includes a compensator cell positioned in both the first opticalpath between the light source and the spatial light modulator and in thesecond optical path between the spatial light modulator and the viewingarea, the compensator cell having a layer of ferroelectric liquidcrystal light modulating medium switchable between a primary and asecondary state so as to act on light in different primary and secondaryways, respectively.
 19. A system according to claim 18 wherein thecompensator cell is positioned optically between the spatial lightmodulator and the passive quarter wave plate.
 20. A system according toclaim 19 wherein the ferroelectric liquid crystal of both the spatiallight modulator and the compensator cell have a buff axis, the buff axisof the spatial light modulator being oriented perpendicular to the buffaxis of the compensator cell.
 21. A system according to claim 20 whereinthe ferroelectric liquid crystal of both the spatial light modulator andthe compensator cell have tilt angles and retardances that are equal toone another.
 22. A system according to claim 21 wherein the spatiallight modulator and the compensator cell both act as quarter waveplates.